I'd say the guitar. So many genres of modern music have this instrument as their staple one. It's also an instrument that many people actually often learn to play voluntarily, and can pick up without too much difficulty. Maybe I just live in a certain day and age, or maybe I just know a few people with a musical ear, but I certainly don't remember any movies portraying kids back in the day fiddling (heh heh) with a violin.
But on that note (ha ha!), I'd have to say the violin and its string kin were a very important breakthrough in music, old as they are. String instruments are very versatile (the instrumental equivalent of the human voice, as my father calls them), and have been used in classical music, movie accompaniments, fiddle rhythms and so forth. The string family is extremely staple in the music world for sure, and even have worked their way into some pop-music.
A third contender would be the drum. Though whether this would count or not, I'm not quite sure, because the drum just might have been the beginning of music anyway. But still, it is a very important part of just about any type of music you can think of (okay, maybe not so much bluegrass...).
One of those three. But they've all made their contributions in some way.